Insertable cardiac monitor

ABSTRACT

Long-term electrocardiographic and physiological monitoring over a period lasting up to several years in duration can be provided through a continuously-recording insertable cardiac monitor. The sensing circuitry and the physical layout of the electrodes are specifically optimized to capture electrical signals from the propagation of low amplitude, relatively low frequency content cardiac action potentials, particularly the P-waves that are generated during atrial activation and storing samples of captured signals. In general, the ICM is intended to be implanted centrally and positioned axially and either over the sternum or slightly to either the left or right of the sternal midline in the parasternal region of the chest.

CROSS-REFERENCE TO RELATED APPLICATIONS

This non-provisional patent application is a continuation of U.S. Pat.No. 10,624,551, issued Apr. 21, 2020, which is a continuation-in-part ofU.S. Pat. No. 10,478,083, issued Nov. 19, 2019, which is continuation ofU.S. Pat. No. 9,730,593, issued Aug. 15, 2017, and further claimspriority under 35 U.S.C. § 119(e) to U.S. Provisional Patentapplication, Ser. No. 61/882,403, filed Sep. 25, 2013, the filing datesof which are claimed and the disclosures of which are incorporated byreference; this present non-provisional patent application is also acontinuation of U.S. Pat. No. 10,624,551, issued Apr. 21, 2020, which isa continuation-in-part of U.S. patent application Ser. No. 15/832,385,filed Dec. 5, 2017, pending, the disclosure of which is incorporated byreference.

FIELD

This application relates in general to electrocardiographic monitoringand, in particular, to an insertable cardiac monitor for use inperforming long term electrocardiographic monitoring.

BACKGROUND

The heart emits electrical signals as a by-product of the propagation ofthe action potentials that trigger depolarization of heart fibers. Anelectrocardiogram (ECG) measures and records such electrical potentialsto visually depict the electrical activity of the heart over time.Conventionally, a standardized set format 12-lead configuration is usedby an ECG machine to record cardiac electrical signals fromwell-established traditional chest locations. Electrodes at the end ofeach lead are placed on the skin over the anterior thoracic region ofthe patient's body to the lower right and to the lower left of thesternum, on the left anterior chest, and on the limbs. Sensed cardiacelectrical activity is represented by PQRSTU waveforms that can beinterpreted post-ECG recordation to derive heart rate and physiology.The P-wave represents atrial electrical activity. The QRSTU componentsrepresent ventricular electrical activity.

An ECG is a tool used by physicians to diagnose heart problems and otherpotential health concerns. An ECG is a snapshot of heart function,typically recorded over 12 seconds, that can help diagnose rate andregularity of heartbeats, effect of drugs or cardiac devices, includingpacemakers and implantable cardioverter-defibrillators (ICDs), andwhether a patient has heart disease. ECGs are used in-clinic duringappointments, and, as a result, are limited to recording only thoseheart-related aspects present at the time of recording. Sporadicconditions that may not show up during a spot ECG recording requireother means to diagnose them. These disorders include fainting orsyncope; rhythm disorders, such as tachyarrhythmias andbradyarrhythmias; apneic episodes; and other cardiac and relateddisorders. Thus, an ECG only provides a partial picture and can beinsufficient for complete patient diagnosis of many cardiac disorders.

Diagnostic efficacy can be improved, when appropriate, through the useof long-term extended ECG monitoring. Recording sufficient ECG, that isof a quality sufficient to be useful in arrhythmia diagnosis, andrelated physiology over an extended period is challenging, and oftenessential to enabling a physician to identify events of potentialconcern. A 30-day observation day period is considered the “goldstandard” of ECG monitoring, yet achieving a 30-day observation dayperiod has proven unworkable because such ECG monitoring systems arearduous to employ, cumbersome to the patient, and excessively costly.Ambulatory monitoring in-clinic is implausible and impracticable.Nevertheless, if a patient's ECG could be recorded in an ambulatorysetting, thereby allowing the patient to engage in activities of dailyliving, the chances of acquiring meaningful information and capturing anabnormal event while the patient is engaged in normal activities becomesmore likely to be achieved.

For instance, the long-term wear of dermal ECG electrodes is complicatedby skin irritation and the inability ECG electrodes to maintaincontinual skin contact after a day or two. Moreover, time, dirt,moisture, and other environmental contaminants, as well as perspiration,skin oil, and dead skin cells from the patient's body, can get betweenan ECG electrode, the non-conductive adhesive used to adhere the ECGelectrode, and the skin's surface. All of these factors adversely affectelectrode adhesion and the quality of cardiac signal recordings.Furthermore, the physical movements of the patient and their clothingimpart various compressional, tensile, and torsional forces on thecontact point of an ECG electrode, especially over long recording times,and an inflexibly fastened ECG electrode will be prone to becomingdislodged. Notwithstanding the cause of electrode dislodgment, dependingupon the type of ECG monitor employed, precise re-placement of adislodged ECG electrode may be essential to ensuring signal capture atthe same fidelity. Moreover, dislodgment may occur unbeknownst to thepatient, making the ECG recordings worthless. Further, some patients mayhave skin that is susceptible to itching or irritation, and the wearingof ECG electrodes can aggravate such skin conditions. Thus, a patientmay want or need to periodically remove or replace ECG electrodes duringa long-term ECG monitoring period, whether to replace a dislodgedelectrode, reestablish better adhesion, alleviate itching or irritation,allow for cleansing of the skin, allow for showering and exercise, orfor other purpose. Such replacement or slight alteration in electrodelocation actually facilitates the goal of recording the ECG signal forlong periods of time.

While subcutaneous ECG monitors can perform monitoring for an extendedperiod of time, up to three years, such subcutaneous ECG monitors,because of their small size, have greater problems of demonstrating aclear and dependable P-wave. The issues related to a tiny atrial voltageare exacerbated by the small size of insertable cardiac monitors (ICMs),the signal processing limits imposed upon them by virtue of theirreduced electrode size, and restricted inter-electrode spacing.Conventional subcutaneous ICMs, as well as most conventional surface ECGmonitors, are notorious for poor visualization of the P-wave, whichremains the primary reason that heart rhythm disorders cannot preciselybe identified today from ICMs. Furthermore, even when physiologicallypresent, the P-wave may not actually appear on an ECG because theP-wave's visibility is strongly dependent upon the signal capturingability of the ECG recording device's sensing circuitry. This situationis further influenced by several factors, including electrodeconfiguration, electrode surface areas and shapes, inter-electrodespacing; where the electrodes are placed on or within the body relativeto the heart's atria. Further, the presence or absence of ambient noiseand the means to limit the ambient noise is a key aspect of whether thelow amplitude atrial signal can be seen.

Conventional ICMs are often used after diagnostic measures when dermalECG monitors fail to identify a suspected arrhythmia. Consequently, whena physician is strongly suspicious of a serious cardiac rhythm disorderthat may have caused loss of consciousness or stroke, for example, thephysician will often proceed to the insertion of an ICM under the skinof the thorax. Although traditionally, the quality of the signal islimited with ICMs with respect to identifying the P-wave, the durationof monitoring is hoped to compensate for poor P-wave recording. Thissituation has led to a dependence on scrutiny of R-wave behavior, suchas RR interval (R-wave-to-R-wave interval) behavior, often used as asurrogate for diagnosing atrial fibrillation, a potential cause ofstroke. To a limited extent, this approach has some degree of value.Nevertheless, better recording of the P-wave would result in asignificant diagnostic improvement, not only in the case of atrialfibrillation, but in a host of other rhythm disorders that can result insyncope or loss of consciousness, like VT or heart block.

The P-wave is the most difficult ECG signal to capture by virtue oforiginating in the low tissue mass atria and having both low voltageamplitude and relatively low frequency content. Notwithstanding thesephysiological constraints, ICMs are popular, albeit limited in theirdiagnostic yield. The few ICMs that are commercially available today,including the Reveal LINQ ICM, manufactured by Medtronic, Inc.,Minneapolis, Minn., the BioMonitor 2 (AF and S versions), manufacturedby Biotronik SE & Co. K G, Berlin, Germany, and the Abbott Confirm RxICM, manufactured by Abbott Laboratories, Chicago, Ill., all areuniformly limited in their abilities to clearly and consistently sense,record, and deliver the P-wave.

Typically, the current realm of ICM devices use a loop recorder wherecumulative ECG data lasting for around an hour is continuallyoverwritten unless an episode of pre-programmed interest occurs or apatient marker is manually triggered. The limited temporal windowafforded by the recordation loop is yet another restriction on theevaluation of the P-wave, and related cardiac morphologies, and furthercompromises diagnostic opportunities.

For instance, Medtronic's Reveal LINQ ICM delivers long-termsubcutaneous ECG monitoring for up to three years, depending onprogramming. The monitor is able to store up to 59 minutes of ECG data,include up to 30 minutes of patient-activated episodes, 27 minutes ofautomatically detected episodes, and two minutes of the longest atrialfibrillation (AF) episode stored since the last interrogation of thedevice. The focus of the device is more directed to recording durationand programming options for recording time and patient interactionsrather than signal fidelity. The Reveal LINQ ICM is intended for generalpurpose ECG monitoring and lacks an engineering focus on P-wavevisualization. Moreover, the device's recording circuitry is intended tosecure the ventricular signal by capturing the R-wave, and is designedto accommodate placement over a broad range of subcutaneous implantationsites, which is usually sufficient if one is focused on the R-wave givenits amplitude and frequency content, but of limited value in capturingthe low-amplitude, low-frequency content P-wave. Finally, electrodespacing, surface areas, and shapes are dictated (and limited) by thephysical size of the monitor's housing which is quite small, anaesthetic choice, but unrealistic with respect to capturing the P-wave.

Similar in design is the titanium housing of Biotronik's BioMonitor 2but with a flexible silicone antenna to mount a distal electrode lead,albeit of a standardized length. This standardized length mollifies, inone parameter only, the concerns of limited inter-electrode spacing andits curbing effect on securing the P-wave. None of the other factorsrelated to P-wave signal revelation are addressed. Therefore the qualityof sensed P-waves reflects a compromise caused by closely-spaced polesthat fail to consistently preserve P-wave fidelity, with the reality ofthe physics imposed problems of signal-to-noise ratio limitationsremaining mostly unaddressed.

Therefore, a need remains for a continuously recording ECG monitorpracticably capable of being worn capable of recording atrial signalsreliably and that is designed for atrial activity recording.

SUMMARY

Physiological monitoring can be provided through a wearable monitor thatincludes two components, a flexible extended wear electrode patch and aremovable reusable monitor recorder. The wearable monitor sits centrally(in the midline) on the patient's chest along the sternum orientedtop-to-bottom. The placement of the wearable monitor in a location atthe sternal midline (or immediately to either side of the sternum), withits unique narrow “hourglass”-like shape, benefits long-term extendedwear by removing the requirement that ECG electrodes be continuallyplaced in the same spots on the skin throughout the monitoring period.Instead, the patient is free to place an electrode patch anywhere withinthe general region of the sternum. In addition, power is providedthrough a battery provided on the electrode patch, which avoids havingto either periodically open the housing of the monitor recorder for thebattery replacement, which also creates the potential for moistureintrusion and human error, or to recharge the battery, which canpotentially take the monitor recorder off line for hours at a time. Inaddition, the electrode patch is intended to be disposable, while themonitor recorder is a reusable component. Thus, each time that theelectrode patch is replaced, a fresh battery is provided for the use ofthe monitor recorder.

Further, long-term electrocardiographic and physiological monitoringover a period lasting up to several years in duration can be providedthrough a continuously-recording subcutaneous insertable cardiac monitor(ICM), such as one described in commonly-owned U.S. patent applicationSer. No. 15/832,385, filed Dec. 5, 2017, pending, the disclosure ofwhich is incorporated by reference. The sensing circuitry and thephysical layout of the electrodes are specifically optimized to captureelectrical signals from the propagation of low amplitude, relatively lowfrequency content cardiac action potentials, particularly the P-wavesthat are generated during atrial activation. In general, the ICM isintended to be implanted centrally and positioned axially and slightlyto either the left or right of the sternal midline in the parasternalregion of the chest. In one embodiment, an insertable cardiac monitor isprovided. An implantable housing is made of a biocompatible materialsuitable for implantation within a living body. A pair of ECG sensingelectrodes is provided on a ventral surface of the implantable housingwith one of the ECG sensing electrodes forming a superior pole on aproximal end of the implantable housing and the other ECG sensingelectrode forming an inferior pole on a distal end of the implantablehousing to capture P-wave signals that are generated during atrialactivation. Electronic circuitry is provided within the implantablehousing and includes a microcontroller operable to execute under modularmicro program control as specified in firmware. An ECG front end circuitis interfaced to the microcontroller and configured to capture thecardiac action potentials of the P-wave signals sensed by the pairing ofthe ECG sensing electrode. A non-volatile memory is electricallyinterfaced with the microcontroller and operable to continuously storesamples of the cardiac action potentials of the P-wave signals. Theforegoing aspects enhance ECG monitoring performance and qualityfacilitating long-term ECG recording, critical to accurate arrhythmiadiagnosis.

Still other embodiments will become readily apparent to those skilled inthe art from the following detailed description, wherein are describedembodiments by way of illustrating the best mode contemplated. As willbe realized, other and different embodiments are possible and theembodiments' several details are capable of modifications in variousobvious respects, all without departing from their spirit and the scope.Accordingly, the drawings and detailed description are to be regarded asillustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are diagrams showing, by way of examples, an extended wearelectrocardiography and physiological sensor monitor, including amonitor recorder in accordance with one embodiment, respectively fittedto the sternal region of a female patient and a male patient.

FIG. 3 is a perspective view showing an extended wear electrode patchwith a monitor recorder in accordance with one embodiment inserted.

FIG. 4 is a perspective view showing the monitor recorder of FIG. 3 .

FIG. 5 is a perspective view showing the extended wear electrode patchof FIG. 3 without a monitor recorder inserted.

FIG. 6 is a bottom plan view of the monitor recorder of FIG. 3 .

FIG. 7 is a top view showing the flexible circuit of the extended wearelectrode patch of FIG. 3 when mounted above the flexible backing.

FIG. 8 is a functional block diagram showing the component architectureof the circuitry of the monitor recorder of FIG. 3 .

FIG. 9 is a functional block diagram showing the circuitry of theextended wear electrode patch of FIG. 3 .

FIG. 10 is a flow diagram showing a monitor recorder-implemented methodfor monitoring ECG data for use in the monitor recorder of FIG. 3 .

FIG. 11 is a graph showing, by way of example, a typical ECG waveform.

FIG. 12 is a diagram showing, by way of example, a subcutaneous P-wavecentric insertable cardiac monitor (ICM) for long termelectrocardiographic monitoring in accordance with one embodiment.

FIGS. 13 and 14 are respectively top and bottom perspective viewsshowing the ICM of FIG. 12 .

FIG. 15 is a bottom perspective view showing the ICM of FIG. 12 inaccordance with a further embodiment.

FIGS. 16 and 17 are respectively top and bottom perspective viewsshowing an ICM in accordance with a still further embodiment.

FIG. 18A and FIG. 18B are plan views showing further electrodeconfigurations.

FIG. 19 is a functional block diagram showing the P-wave focusedcomponent architecture of the circuitry 280 of the ICM 212 of FIG. 12 .

DETAILED DESCRIPTION

Physiological monitoring can be provided through a wearable monitor thatincludes two components, a flexible extended wear electrode patch and aremovable reusable monitor recorder. FIGS. 1 and 2 are diagrams showing,by way of examples, an extended wear electrocardiography andphysiological sensor monitor 12, including a monitor recorder 14 inaccordance with one embodiment, respectively fitted to the sternalregion of a female patient 10 and a male patient 11. The wearablemonitor 12 sits centrally (in the midline) on the patient's chest alongthe sternum 13 oriented top-to-bottom with the monitor recorder 14preferably situated towards the patient's head. In a further embodiment,the orientation of the wearable monitor 12 can be correctedpost-monitoring, as further described infra. The electrode patch 15 isshaped to fit comfortably and conformal to the contours of the patient'schest approximately centered on the sternal midline 16 (or immediatelyto either side of the sternum 13). The distal end of the electrode patch15 extends towards the Xiphoid process and, depending upon the patient'sbuild, may straddle the region over the Xiphoid process. The proximalend of the electrode patch 15, located under the monitor recorder 14, isbelow the manubrium and, depending upon patient's build, may straddlethe region over the manubrium.

The placement of the wearable monitor 12 in a location at the sternalmidline 16 (or immediately to either side of the sternum 13)significantly improves the ability of the wearable monitor 12 tocutaneously sense cardiac electric signals, particularly the P-wave (oratrial activity) and, to a lesser extent, the QRS interval signals inthe ECG waveforms that indicate ventricular activity. The sternum 13overlies the right atrium of the heart and the placement of the wearablemonitor 12 in the region of the sternal midline 13 puts the ECGelectrodes of the electrode patch 15 in a location better adapted tosensing and recording P-wave signals than other placement locations,say, the upper left pectoral region. In addition, placing the lower orinferior pole (ECG electrode) of the electrode patch 15 over (or near)the Xiphoid process facilitates sensing of right ventricular activityand provides superior recordation of the QRS interval.

During use, the electrode patch 15 is first adhesed to the skin alongthe sternal midline 16 (or immediately to either side of the sternum13). A monitor recorder 14 is then snapped into place on the electrodepatch 15 to initiate ECG monitoring. FIG. 3 is a perspective viewshowing an extended wear electrode patch 15 with a monitor recorder 14in accordance with one embodiment inserted. The body of the electrodepatch 15 is preferably constructed using a flexible backing 20 formed asan elongated strip 21 of wrap knit or similar stretchable material witha narrow longitudinal mid-section 23 evenly tapering inward from bothsides. A pair of cut-outs 22 between the distal and proximal ends of theelectrode patch 15 create a narrow longitudinal midsection 23 or“isthmus” and defines an elongated “hourglass”-like shape, when viewedfrom above.

The electrode patch 15 incorporates features that significantly improvewearability, performance, and patient comfort throughout an extendedmonitoring period. During wear, the electrode patch 15 is susceptible topushing, pulling, and torqueing movements, including compressional andtorsional forces when the patient bends forward, and tensile andtorsional forces when the patient leans backwards. To counter thesestress forces, the electrode patch 15 incorporates strain and crimpreliefs, such as described in commonly-assigned U.S. Patent, entitled“Extended Wear Electrocardiography Patch,” U.S. Pat. No. 9,545,204,issued on Jan. 17, 2017, the disclosure of which is incorporated byreference. In addition, the cut-outs 22 and longitudinal midsection 23help minimize interference with and discomfort to breast tissue,particularly in women (and gynecomastic men). The cut-outs 22 andlongitudinal midsection 23 further allow better conformity of theelectrode patch 15 to sternal bowing and to the narrow isthmus of flatskin that can occur along the bottom of the intermammary cleft betweenthe breasts, especially in buxom women. The cut-outs 22 and longitudinalmidsection 23 help the electrode patch 15 fit nicely between a pair offemale breasts in the intermammary cleft. Still other shapes, cut-outsand conformities to the electrode patch 15 are possible.

The monitor recorder 14 removably and reusably snaps into anelectrically non-conductive receptacle 25 during use. The monitorrecorder 14 contains electronic circuitry for recording and storing thepatient's electrocardiography as sensed via a pair of ECG electrodesprovided on the electrode patch 15, as further described infra beginningwith reference to FIG. 8 . The non-conductive receptacle 25 is providedon the top surface of the flexible backing 20 with a retention catch 26and tension clip 27 molded into the non-conductive receptacle 25 toconformably receive and securely hold the monitor recorder 14 in place.

The monitor recorder 14 includes a sealed housing that snaps into placein the non-conductive receptacle 25. FIG. 4 is a perspective viewshowing the monitor recorder 14 of FIG. 3 . The sealed housing 50 of themonitor recorder 14 intentionally has a rounded isoscelestrapezoidal-like shape 52, when viewed from above, such as described incommonly-assigned U.S. Design Patent, entitled “ElectrocardiographyMonitor,” U.S. Design No. D717955, issued on Nov. 18, 2014, thedisclosure of which is incorporated by reference. The edges 51 along thetop and bottom surfaces are rounded for patient comfort. The sealedhousing 50 is approximately 47 mm long, 23 mm wide at the widest point,and 7 mm high, excluding a patient-operable tactile-feedback button 55.The sealed housing 50 can be molded out of polycarbonate, ABS, or analloy of those two materials. The button 55 is waterproof and thebutton's top outer surface is molded silicon rubber or similar softpliable material. A retention detent 53 and tension detent 54 are moldedalong the edges of the top surface of the housing 50 to respectivelyengage the retention catch 26 and the tension clip 27 molded intonon-conductive receptacle 25. Other shapes, features, and conformitiesof the sealed housing 50 are possible.

The electrode patch 15 is intended to be disposable. The monitorrecorder 14, however, is reusable and can be transferred to successiveelectrode patches 15 to ensure continuity of monitoring. The placementof the wearable monitor 12 in a location at the sternal midline 16 (orimmediately to either side of the sternum 13) benefits long-termextended wear by removing the requirement that ECG electrodes becontinually placed in the same spots on the skin throughout themonitoring period. Instead, the patient is free to place an electrodepatch 15 anywhere within the general region of the sternum 13.

As a result, at any point during ECG monitoring, the patient's skin isable to recover from the wearing of an electrode patch 15, whichincreases patient comfort and satisfaction, while the monitor recorder14 ensures ECG monitoring continuity with minimal effort. A monitorrecorder 14 is merely unsnapped from a worn out electrode patch 15, theworn out electrode patch 15 is removed from the skin, a new electrodepatch 15 is adhered to the skin, possibly in a new spot immediatelyadjacent to the earlier location, and the same monitor recorder 14 issnapped into the new electrode patch 15 to reinitiate and continue theECG monitoring.

During use, the electrode patch 15 is first adhered to the skin in thesternal region. FIG. 5 is a perspective view showing the extended wearelectrode patch 15 of FIG. 3 without a monitor recorder 14 inserted. Aflexible circuit 32 is adhered to each end of the flexible backing 20. Adistal circuit trace 33 and a proximal circuit trace (not shown)electrically couple ECG electrodes (not shown) to a pair of electricalpads 34. The electrical pads 34 are provided within a moisture-resistantseal 35 formed on the bottom surface of the non-conductive receptacle25. When the monitor recorder 14 is securely received into thenon-conductive receptacle 25, that is, snapped into place, theelectrical pads 34 interface to electrical contacts (not shown)protruding from the bottom surface of the monitor recorder 14, and themoisture-resistant seal 35 enables the monitor recorder 14 to be worn atall times, even during bathing or other activities that could expose themonitor recorder 14 to moisture.

In addition, a battery compartment 36 is formed on the bottom surface ofthe non-conductive receptacle 25, and a pair of battery leads (notshown) electrically interface the battery to another pair of theelectrical pads 34. The battery contained within the battery compartment35 can be replaceable, rechargeable or disposable.

The monitor recorder 14 draws power externally from the battery providedin the non-conductive receptacle 25, thereby uniquely obviating the needfor the monitor recorder 14 to carry a dedicated power source. FIG. 6 isa bottom plan view of the monitor recorder 14 of FIG. 3 . A cavity 58 isformed on the bottom surface of the sealed housing 50 to accommodate theupward projection of the battery compartment 36 from the bottom surfaceof the non-conductive receptacle 25, when the monitor recorder 14 issecured in place on the non-conductive receptacle 25. A set ofelectrical contacts 56 protrude from the bottom surface of the sealedhousing 50 and are arranged in alignment with the electrical pads 34provided on the bottom surface of the non-conductive receptacle 25 toestablish electrical connections between the electrode patch 15 and themonitor recorder 14. In addition, a seal coupling 57 circumferentiallysurrounds the set of electrical contacts 56 and securely mates with themoisture-resistant seal 35 formed on the bottom surface of thenon-conductive receptacle 25.

The placement of the flexible backing 20 on the sternal midline 16 (orimmediately to either side of the sternum 13) also helps to minimize theside-to-side movement of the wearable monitor 12 in the left- andright-handed directions during wear. To counter the dislodgment of theflexible backing 20 due to compressional and torsional forces, a layerof non-irritating adhesive, such as hydrocolloid, is provided at leastpartially on the underside, or contact, surface of the flexible backing20, but only on the distal end 30 and the proximal end 31. As a result,the underside, or contact surface of the longitudinal midsection 23 doesnot have an adhesive layer and remains free to move relative to theskin. Thus, the longitudinal midsection 23 forms a crimp relief thatrespectively facilitates compression and twisting of the flexiblebacking 20 in response to compressional and torsional forces. Otherforms of flexible backing crimp reliefs are possible. Unlike theflexible backing 20, the flexible circuit 32 is only able to bend andcannot stretch in a planar direction. The flexible circuit 32 can beprovided either above or below the flexible backing 20. FIG. 7 is a topview showing the flexible circuit 32 of the extended wear electrodepatch 15 of FIG. 3 when mounted above the flexible backing 20. A distalECG electrode 38 and proximal ECG electrode 39 are respectively coupledto the distal and proximal ends of the flexible circuit 32. A strainrelief 40 is defined in the flexible circuit 32 at a location that ispartially underneath the battery compartment 36 when the flexiblecircuit 32 is affixed to the flexible backing 20. The strain relief 40is laterally extendable to counter dislodgment of the ECG electrodes 38,39 due to tensile and torsional forces. A pair of strain relief cutouts41 partially extend transversely from each opposite side of the flexiblecircuit 32 and continue longitudinally towards each other to define in‘S’-shaped pattern, when viewed from above. The strain reliefrespectively facilitates longitudinal extension and twisting of theflexible circuit 32 in response to tensile and torsional forces. Otherforms of circuit board strain relief are possible. ECG monitoring andother functions performed by the monitor recorder 14 are providedthrough a micro controlled architecture. FIG. 8 is a functional blockdiagram showing the component architecture of the circuitry 60 of themonitor recorder 14 of FIG. 3 . The circuitry 60 is externally poweredthrough a battery provided in the non-conductive receptacle 25 (shown inFIG. 5 ). Both power and raw ECG signals, which originate in the pair ofECG electrodes 38, 39 (shown in FIG. 7 ) on the distal and proximal endsof the electrode patch 15, are received through an external connector 65that mates with a corresponding physical connector on the electrodepatch 15. The external connector 65 includes the set of electricalcontacts 56 that protrude from the bottom surface of the sealed housing50 and which physically and electrically interface with the set of pads34 provided on the bottom surface of the non-conductive receptacle 25.The external connector includes electrical contacts 56 for datadownload, microcontroller communications, power, analog inputs, and aperipheral expansion port. The arrangement of the pins on the electricalconnector 65 of the monitor recorder 14 and the device into which themonitor recorder 14 is attached, whether an electrode patch 15 ordownload station (not shown), follow the same electrical pin assignmentconvention to facilitate interoperability. The external connector 65also serves as a physical interface to a download station that permitsthe retrieval of stored ECG monitoring data, communication with themonitor recorder 14, and performance of other functions.

Operation of the circuitry 60 of the monitor recorder 14 is managed by amicrocontroller 61. The micro-controller 61 includes a program memoryunit containing internal flash memory that is readable and writeable.The internal flash memory can also be programmed externally. Themicro-controller 61 draws power externally from the battery provided onthe electrode patch 15 via a pair of the electrical contacts 56. Themicrocontroller 61 connects to the ECG front end circuit 63 thatmeasures raw cutaneous electrical signals and generates an analog ECGsignal representative of the electrical activity of the patient's heartover time.

The circuitry 60 of the monitor recorder 14 also includes a flash memory62, which the micro-controller 61 uses for storing ECG monitoring dataand other physiology and information. The flash memory 62 also drawspower externally from the battery provided on the electrode patch 15 viaa pair of the electrical contacts 56. Data is stored in a serial flashmemory circuit, which supports read, erase and program operations over acommunications bus. The flash memory 62 enables the microcontroller 61to store digitized ECG data. The communications bus further enables theflash memory 62 to be directly accessed externally over the externalconnector 65 when the monitor recorder 14 is interfaced to a downloadstation.

The circuitry 60 of the monitor recorder 14 further includes anactigraphy sensor 64 implemented as a 3-axis accelerometer. Theaccelerometer may be configured to generate interrupt signals to themicrocontroller 61 by independent initial wake up and free fall events,as well as by device position. In addition, the actigraphy provided bythe accelerometer can be used during post-monitoring analysis to correctthe orientation of the monitor recorder 14 if, for instance, the monitorrecorder 14 has been inadvertently installed upside down, that is, withthe monitor recorder 14 oriented on the electrode patch 15 towards thepatient's feet, as well as for other event occurrence analyses.

The microcontroller 61 includes an expansion port that also utilizes thecommunications bus. External devices, separately drawing powerexternally from the battery provided on the electrode patch 15 or othersource, can interface to the microcontroller 61 over the expansion portin half duplex mode. For instance, an external physiology sensor can beprovided as part of the circuitry 60 of the monitor recorder 14, or canbe provided on the electrode patch 15 with communication with themicro-controller 61 provided over one of the electrical contacts 56. Thephysiology sensor can include an SpO₂ sensor, blood pressure sensor,temperature sensor, respiratory rate sensor, glucose sensor, airflowsensor, volumetric pressure sensing, or other types of sensor ortelemetric input sources. In a further embodiment, a wireless interfacefor interfacing with other wearable (or implantable) physiologymonitors, as well as data offload and programming, can be provided aspart of the circuitry 60 of the monitor recorder 14, or can be providedon the electrode patch 15 with communication with the micro-controller61 provided over one of the electrical contacts 56.

Finally, the circuitry 60 of the monitor recorder 14 includespatient-interfaceable components, including a tactile feedback button66, which a patient can press to mark events or to perform otherfunctions, and a buzzer 67, such as a speaker, magnetic resonator orpiezoelectric buzzer. The buzzer 67 can be used by the microcontroller61 to output feedback to a patient such as to confirm power up andinitiation of ECG monitoring. Still other components as part of thecircuitry 60 of the monitor recorder 14 are possible. While the monitorrecorder 14 operates under micro control, most of the electricalcomponents of the electrode patch 15 operate passively. FIG. 9 is afunctional block diagram showing the circuitry 70 of the extended wearelectrode patch 15 of FIG. 3 . The circuitry 70 of the electrode patch15 is electrically coupled with the circuitry 60 of the monitor recorder14 through an external connector 74. The external connector 74 isterminated through the set of pads 34 provided on the bottom of thenon-conductive receptacle 25, which electrically mate to correspondingelectrical contacts 56 protruding from the bottom surface of the sealedhousing 50 to electrically interface the monitor recorder 14 to theelectrode patch 15.

The circuitry 70 of the electrode patch 15 performs three primaryfunctions. First, a battery 71 is provided in a battery compartmentformed on the bottom surface of the non-conductive receptacle 25. Thebattery 71 is electrically interfaced to the circuitry 60 of the monitorrecorder 14 as a source of external power. The unique provisioning ofthe battery 71 on the electrode patch 15 provides several advantages.First, the locating of the battery 71 physically on the electrode patch15 lowers the center of gravity of the overall wearable monitor 12 andthereby helps to minimize shear forces and the effects of movements ofthe patient and clothing. Moreover, the housing 50 of the monitorrecorder 14 is sealed against moisture and providing power externallyavoids having to either periodically open the housing 50 for the batteryreplacement, which also creates the potential for moisture intrusion andhuman error, or to recharge the battery, which can potentially take themonitor recorder 14 off line for hours at a time. In addition, theelectrode patch 15 is intended to be disposable, while the monitorrecorder 14 is a reusable component. Each time that the electrode patch15 is replaced, a fresh battery is provided for the use of the monitorrecorder 14, which enhances ECG monitoring performance quality andduration of use. Finally, the architecture of the monitor recorder 14 isopen, in that other physiology sensors or components can be added byvirtue of the expansion port of the microcontroller 61. Requiring thoseadditional sensors or components to draw power from a source external tothe monitor recorder 14 keeps power considerations independent of themonitor recorder 14. Thus, a battery of higher capacity could beintroduced when needed to support the additional sensors or componentswithout effecting the monitor recorders circuitry 60.

Second, the pair of ECG electrodes 38, 39 respectively provided on thedistal and proximal ends of the flexible circuit 32 are electricallycoupled to the set of pads 34 provided on the bottom of thenon-conductive receptacle 25 by way of their respective circuit traces33, 37. The signal ECG electrode 39 includes a protection circuit 72,which is an inline resistor that protects the patient from excessiveleakage current.

Last, in a further embodiment, the circuitry 70 of the electrode patch15 includes a cryptographic circuit 73 to authenticate an electrodepatch 15 for use with a monitor recorder 14. The cryptographic circuit73 includes a device capable of secure authentication and validation.The cryptographic device 73 ensures that only genuine, non-expired,safe, and authenticated electrode patches 15 are permitted to providemonitoring data to a monitor recorder 14.

The monitor recorder 14 continuously monitors the patient's heart rateand physiology. FIG. 10 is a flow diagram showing a monitorrecorder-implemented method 100 for monitoring ECG data for use in themonitor recorder 14 of FIG. 3 . Initially, upon being connected to theset of pads 34 provided with the non-conductive receptacle 25 when themonitor recorder 14 is snapped into place, the microcontroller 61executes a power up sequence (step 101). During the power up sequence,the voltage of the battery 71 is checked, the state of the flash memory62 is confirmed, both in terms of operability check and availablecapacity, and microcontroller operation is diagnostically confirmed. Ina further embodiment, an authentication procedure between themicrocontroller 61 and the electrode patch 15 are also performed.

Following satisfactory completion of the power up sequence, an iterativeprocessing loop (steps 102-109) is continually executed by themicrocontroller 61. During each iteration (step 102) of the processingloop, the ECG frontend 63 (shown in FIG. 8 ) continually senses thecutaneous ECG electrical signals (step 103) via the ECG electrodes 38,29 and is optimized to maintain the integrity of the P-wave. A sample ofthe ECG signal is read (step 104) by the microcontroller 61 by samplingthe analog ECG signal output front end 63. FIG. 11 is a graph showing,by way of example, a typical ECG waveform 110. The x-axis representstime in approximate units of tenths of a second. The y-axis representscutaneous electrical signal strength in approximate units of millivolts.The P-wave 111 has a smooth, normally upward, that is, positive,waveform that indicates atrial depolarization. The QRS complex usuallybegins with the downward deflection of a Q wave 112, followed by alarger upward deflection of an R-wave 113, and terminated with adownward waveform of the S wave 114, collectively representative ofventricular depolarization. The T wave 115 is normally a modest upwardwaveform, representative of ventricular depolarization, while the U wave116, often not directly observable, indicates the recovery period of thePurkinje conduction fibers.

Sampling of the R-to-R interval enables heart rate informationderivation. For instance, the R-to-R interval represents the ventricularrate and rhythm, while the P-to-P interval represents the atrial rateand rhythm. Importantly, the PR interval is indicative ofatrioventricular (AV) conduction time and abnormalities in the PRinterval can reveal underlying heart disorders, thus representinganother reason why the P-wave quality achievable by the extended wearambulatory electrocardiography and physiological sensor monitordescribed herein is medically unique and important. The long-termobservation of these ECG indicia, as provided through extended wear ofthe wearable monitor 12, provides valuable insights to the patient'scardiac function and overall well-being.

Each sampled ECG signal, in quantized and digitized form, is temporarilystaged in buffer (step 105), pending compression preparatory to storagein the flash memory 62 (step 106). Following compression, the compressedECG digitized sample is again buffered (step 107), then written to theflash memory 62 (step 108) using the communications bus. Processingcontinues (step 109), so long as the monitoring recorder 14 remainsconnected to the electrode patch 15 (and storage space remains availablein the flash memory 62), after which the processing loop is exited andexecution terminates. Still other operations and steps are possible.

In a further embodiment, the method 100 described above with referenceto FIG. 10 can also be implemented by a continuously-recordingsubcutaneous insertable cardiac monitor (ICM), such as one described incommonly-owned U.S. patent application Ser. No. 15/832,385, filed Dec.5, 2017, pending, the disclosure of which is incorporated by reference.The ICM can be used for conducting a long-term electrocardiographic andphysiological monitoring over a period lasting up to several years induration. FIG. 12 is a diagram showing, by way of example, asubcutaneous P-wave centric ICM 212 for long term electrocardiographicmonitoring in accordance with one embodiment. The ICM 212 is implantedin the parasternal region 211 of a patient 10. The sensing circuitry andcomponents, compression algorithms, and the physical layout of theelectrodes are specifically optimized to capture electrical signals fromthe propagation of low amplitude, relatively low frequency contentcardiac action potentials, particularly the P-waves generated duringatrial activation. The position and placement of the ICM 212 coupled toengineering considerations that optimize the ICM's sensing circuitry,discussed infra, aid in demonstrating the P-wave clearly.

Implantation of a P-wave centric ICM 212 in the proper subcutaneous sitefacilitates the recording of high quality ECG data with a gooddelineation of the P-wave. In general, the ICM 212 is intended to beimplanted anteriorly and be positioned axially and slightly to eitherthe right or left of the sternal midline in the parasternal region 211of the chest, or if sufficient subcutaneous fat exists, directly overthe sternum. Optimally, the ICM 212 is implanted in a location leftparasternally to bridge the left atrial appendage. However, eitherlocation to the right or left of the sternal midline is acceptable;placement of the device, if possible, should bridge the vertical heightof the heart, which lies underneath the sternum 203, thereby placing theICM 212 in close proximity to the anterior right atrium and the leftatrial appendage that lie immediately beneath.

The ICM 212 is shaped to fit comfortably within the body under the skinand to conform to the contours of the patient's parasternal region 211when implanted immediately to either side of the sternum 203, but couldbe implanted in other locations of the body. In most adults, theproximal end 213 of the ICM 212 is generally positioned below themanubrium 8 but, depending upon patient's vertical build, the ICM 212may actually straddle the region over the manubrium 8. The distal end214 of the ICM 212 generally extends towards the xiphoid process 9 andlower sternum but, depending upon the patient's build, may actuallystraddle the region over or under the xiphoid process 9, lower sternumand upper abdomen.

Although internal tissues, body structures, and tissue boundaries canadversely affect the current strength and signal fidelity of all bodysurface potentials, subsurface low amplitude cardiac action potentials,particularly P-wave signals with a normative amplitude of less than 0.25millivolts (mV) and a normative duration of less than 120 milliseconds(ms), are most apt to be negatively impacted by these factors. Theatria, which generate the P wave, are mostly located posteriorly withinthe thoracic cavity (with the exception of the anterior right atrium,right atrial appendage and left atrial appendage). The majority of theleft atrium constitutes the portion of the heart furthest away from thesurface of the skin on the chest and harbors the atrial tissue mostlikely to be the source of serious arrhythmias, like atrialfibrillation. Conversely, the ventricles, which generate largeramplitude signals, are located anteriorly as in the case of the anteriorright ventricle and most of the anterior left ventricle situatedrelatively close to the skin surface of the central and left anteriorchest. These factors, together with larger size and more powerfulimpulse generation from the ventricles, contribute to the relativelylarger amplitudes of ventricular waveforms.

Nevertheless, as explained supra, both the P-wave and the R-wave arerequired for the physician to make a proper rhythm diagnosis from thedozens of arrhythmias that can occur. Yet, the quality of P-waves ismore susceptible to weakening from distance and the intervening tissuesand structures and from signal attenuation and signal processing thanthe high voltage waveforms associated with ventricular activation. Theadded value of avoiding further signal attenuation resulting from dermalimpedance makes a subcutaneous P-wave centric ICM even more likely tomatch, or even outperform dermal ambulatory monitors designed toanalogous engineering considerations and using similar sensing circuitryand components, compression algorithms, and physical layout ofelectrodes, such as described in U.S. Pat. No. 9,545,204, issued Jan.217, 20217 to Bishay et al.; U.S. Pat. No. 9,730,593, issued Aug. 15,20217 to Felix et al.; U.S. Pat. No. 9,700,227, issued Jul. 11, 20217 toBishay et al.; U.S. Pat. No. 9,7217,433, issued Aug. 1, 20217 to Felixet al.; and U.S. Pat. No. 9,615,763, issued Apr. 11, 20217 to Felix etal., the disclosures of which are incorporated by reference.

The ICM 212 can be implanted in the patient's chest using, for instance,a minimally invasive subcutaneous implantation instrument or othersuitable surgical implement. The ICM 212 is positioned slightly to theright or left of midline, covering the center third of the chest,roughly between the second and sixth ribs, approximately spanningbetween the level of the manubrium 8 and the level of the xiphoidprocess 9 on the inferior border of the sternum 203, depending upon thevertical build of the patient 210.

During monitoring, the amplitude and strength of action potentialssensed by an ECG devices, including dermal ECG monitors and ICMs, can beaffected to varying degrees by cardiac, cellular, extracellular, vectorof current flow, and physical factors, like obesity, dermatitis, lungdisease, large breasts, and high impedance skin, as can occur indark-skinned individuals. Performing ECG sensing subcutaneously in theparasternal region 211 significantly improves the ability of the ICM 212to counter some of the effects of these factors, particularly high skinimpedance and impedance from subcutaneous fat. Thus, the ICM 212exhibits superior performance when compared to conventional dermal ECGmonitors to existing implantable loop recorders, ICMs, and other formsof implantable monitoring devices by virtue of its engineering andproven P-wave documentation above the skin, as discussed in W. M. Smithet al., “Comparison of diagnostic value using a small, single channel,P-wave centric sternal ECG monitoring patch with a standard 3-leadHolter system over 24 hours,” Am. Heart J., Mar. 20217; 2185:67-73, thedisclosure of which is incorporated by reference.

Moreover, the sternal midline implantation location in the parasternalregion 211 allows the ICM's electrodes to record an ECG of optimalsignal quality from a location immediately above the strongestsignal-generating aspects of the atrial. Signal quality is improvedfurther in part because cardiac action potential propagation travelssimultaneously along a north-to-south and right-to-left vector,beginning high in the right atrium and ultimately ending in theposterior and lateral region of the left ventricle. Cardiacdepolarization originates high in the right atrium in the SA node beforeconcurrently spreading leftward towards the left atrium and inferiorlytowards the atrioventricular (AV) node. On the proximal end 213, the ECGelectrodes of the ICM 212 are subcutaneously positioned with the upperor superior pole (ECG electrode) slightly to the right or left of thesternal midline in the region of the manubrium 8 and, on the distal end214, the lower or inferior pole (ECG electrode) is similarly situatedslightly to the right or left of the sternal midline in the region ofthe xiphoid process 9 and lower sternum 203. The ECG electrodes of theICM 212 are placed primarily in a north-to-south orientation along thesternum 203 that corresponds to the north-to-south waveform vectorexhibited during atrial activation. This orientation corresponds to theaVF lead used in a conventional 12-lead ECG that is used to sensepositive or upright P-waves. In addition, the electrode spacing and theelectrodes' shapes and surface areas mimic the electrodes used in theICM's dermal cousin, designed as part of the optimal P-wave sensingelectrode configuration, such as provided with the dermal ambulatorymonitors cited supra.

Despite the challenges faced in capturing low amplitude cardiac actionpotentials, the ICM 212 is able to operate effectively using only twoelectrodes that are strategically sized and placed in locations ideallysuited to high fidelity P-wave signal acquisition. This approach hasbeen shown to clinically outperform more typical multi-lead monitorsbecause of the improved P-wave clarity, as discussed in W. M. Smith etal., cited supra. FIGS. 13 and 14 are respectively top and bottomperspective views showing the ICM 212 of FIG. 1 . Physically, the ICM212 is constructed with a hermetically sealed implantable housing 215with at least one ECG electrode forming a superior pole on the proximalend 213 and at least one ECG electrode forming an inferior pole on thedistal end 214.

When implanted, the housing 215 is oriented most cephalad. The housing215 is constructed of titanium, stainless steel or other biocompatiblematerial. The housing 215 contains the sensing, recordation andinterfacing circuitry of the ICM 212, plus a long life battery. Awireless antenna is integrated into or within the housing 215 and can bepositioned to wrap around the housing's internal periphery or locationsuited to signal reception. Other wireless antenna placement orintegrations are possible.

Physically, the ICM 212 has four ECG electrodes 216, 217, 218, 219.There could also be additional ECG electrodes, as discussed infra. TheECG electrodes include two ventral (or dorsal) ECG electrodes 218, 219and two wraparound ECG electrodes 216, 217. One ventral ECG electrode218 is formed on the proximal end 213 and one ventral ECG electrode 219is formed on the distal end 214. One wraparound ECG electrode 216 isformed circumferentially about the proximal end 213 and one wraparoundECG electrode 217 is formed circumferentially about the distal end 214.Each wraparound ECG electrode 216, 217 is electrically insulated fromits respective ventral ECG electrode 218, 219 by a periphery 220, 221.

The four ECG electrodes 216, 217, 218, 219 are programmaticallycontrolled by a microcontroller through onboard firmware programming toenable a physician to choose from several different electrodeconfigurations that vary the electrode surface areas, shapes, andinter-electrode spacing. The sensing circuitry can be programmed, eitherpre-implant or in situ, to use different combinations of the availableECG electrodes (and thereby changing electrode surface areas, shapes,and inter-electrode spacing), including pairing the two ventral ECGelectrodes 216, 217, the two wraparound ECG electrodes 218, 219, or oneventral ECG electrode 216, 217 with one wraparound ECG electrode 218,219 located on the opposite end of the housing 215. In addition, theperiphery 220, 221 can be programmatically controlled to logicallycombine the wraparound ECG electrode 216, 217 on one end of the ICM 212with its corresponding ventral ECG electrode 218, 219 to form a singlevirtual ECG electrode with larger surface area and shape. (Althoughelectronically possible, the two ECG electrodes that are only on one endof the ICM 212, for instance, wraparound ECG electrode 216 and ventralECG electrode 218, could be paired; however, the minimal inter-electrodespacing would likely yield a signal of poor fidelity in mostsituations.)

In a further embodiment, the housing 215 and contained circuitry can beprovided as a standalone ICM core assembly to which a pair of compatibleECG electrodes can be operatively coupled to form a full implantable ICMdevice.

Other ECG electrode configurations are possible. For instance,additional ECG electrodes can be provided to increase the number ofpossible electrode configurations, all of which are to ensure betterP-wave resolution. FIG. 15 is a bottom perspective view showing the ICM212 of FIG. 12 in accordance with a further embodiment. An additionalpair of ventral ECG electrodes 222, 223 are included on the housing'sventral surface. These ventral ECG electrodes 222, 223 are spaced closertogether than the ventral ECG electrodes 218, 219 on the ends of thehousing 215 and a physician can thus choose to pair the two innerventral ECG electrodes 222, 223 by themselves to allow for minimalelectrode-to-electrode spacing, or with the other ECG electrodes 216,217, 218, 219 to vary electrode surface areas, shapes, andinter-electrode spacing even further to explore optimal configurationsto acquire the P-wave.

Other housing configurations of the ICM are possible. For instance, thehousing of the ICM can be structured to enhance long term comfort andfitment, and to accommodate a larger long life battery or more circuitryor features, including physiologic sensors, to provide additionalfunctionality. FIGS. 16 and 17 are respectively top and bottomperspective views showing an ICM 230 in accordance with a still furtherembodiment. The ICM 230 has a housing 231 with a tapered extension 232that is terminated on the distal end with an electrode 234. On aproximal end, the housing 231 includes a pair of ECG electrodeselectrically insulated by a periphery 237 that include a ventral ECGelectrode 233 and a wraparound ECG electrode 234. In addition, a ventralECG electrode 236 is oriented on the housing's distal end before thetapered extension 232. Still other housing structures and electrodeconfigurations are possible.

In general, the basic electrode layout is sufficient to sense cardiacaction potentials in a wide range of patients. Differences in thoracictissue density and skeletal structure from patient to patient, though,can affect the ability of the sensing electrodes to efficaciouslycapture action potential signals, yet the degree to which signalacquisition is affected may not be apparent until after an ICM has beenimplanted and deployed, when the impacts of the patient's physicalconstitution and his patterns of mobility and physical movement on ICMmonitoring can be fully assessed.

In further embodiments, the electrodes can be configured post-implant toallow the ICM to better adapt to a particular patient's physiology. Forinstance, electrode configurations having more than two sensingelectrodes are possible. FIG. 18A and FIG. 18B are plan views showingfurther electrode configurations. Referring first to FIG. 18A, a singledisc ECG electrode 240 could be bifurcated to form a pair of half-circleECG electrodes 241, 242 that could be programmatically selected orcombined to accommodate a particular patients ECG signal characteristicspost-ICM implant. Referring next to FIG. 18B, a single disc ECGelectrode 245 could be divided into three sections, a pair ofcrescent-shaped ECG electrodes 246, 247 surrounding a centralsemicircular ECG electrode 248 that could similarly be programmaticallyselected or combined. Still other ECG electrode configurations arepossible.

ECG monitoring and other functions performed by the ICM 212 are providedthrough a micro controlled architecture. FIG. 19 is a functional blockdiagram showing the P-wave focused component architecture of thecircuitry 280 of the ICM 212 of FIG. 12 . The circuitry 280 is poweredthrough the long life battery 21 provided in the housing 215, which canbe a direct current battery. Operation of the circuitry 280 of the ICM212 is managed by a microcontroller 281, such as the EFM32 Tiny Gecko32-bit microcontroller, manufactured by Silicon Laboratories Inc.,Austin, Tex. The microcontroller 281 has flexible energy managementmodes and includes a direct memory access controller and built-inanalog-to-digital and digital-to-analog converters (ADC and DAC,respectively). The microcontroller 281 also includes a program memoryunit containing internal flash memory (not shown) that is readable,writeable, and externally programmable.

The microcontroller 281 operates under modular micro program control asspecified in firmware stored in the internal flash memory. Themicrocontroller 281 draws power from the battery provided in the housing215 and connects to the ECG front end circuit 63. The front end circuit63 measures raw subcutaneous electrical signals using a driven referencesignal that eliminates common mode noise, as further described infra.

The circuitry 280 of the ICM 212 also includes a flash memory 282external to the microcontroller 281, which the microcontroller 281 usesfor continuously storing samples of ECG monitoring signal data and otherphysiology, such as respiratory rate, blood oxygen saturation level(SpO₂), blood pressure, temperature sensor, and physical activity, anddevice and related information. The flash memory 282 also draws powerfrom the battery provided in the housing 215. Data is stored in a serialflash memory circuit, which supports read, erase and program operationsover a communications bus. The flash memory 282 enables themicrocontroller 281 to store digitized ECG data. The communications busfurther enables the flash memory 282 to be directly accessed wirelesslythrough a transceiver 285 coupled to an antenna 217 built into (orprovided with) the housing 215. The transceiver 285 can be used forwirelessly interfacing over Bluetooth or other types of wirelesstechnologies for exchanging data over a short distance with a pairedmobile device, including smartphones and smart watches, that aredesigned to communicate over a public communications infrastructure,such as a cellular communications network, and, in a further embodiment,other wearable (or implantable) physiology monitors, such as activitytrackers worn on the wrist or body. Other types of device pairings arepossible, including with a desktop computer or purpose-built bedsidemonitor. The transceiver 285 can be used to offload stored ECGmonitoring data and other physiology data and information and for devicefirmware reprogramming. In a further embodiment, the flash memory 282can be accessed through an inductive coupling (not shown).

The microcontroller 281 includes functionality that enables theacquisition of samples of analog ECG signals, which are converted into adigital representation, implementing the method 100 described supra withreference to FIG. 10 . In one mode, the microcontroller 281 implements aloop recorder feature that will acquire, sample, digitize, signalprocess, and store digitized ECG data into available storage locationsin the flash memory 282 until all memory storage locations are filled,after which existing stored digitized ECG data will either beoverwritten through a sliding window protocol, albeit at the cost ofpotentially losing the stored data that was overwritten, if notpreviously downloaded, or transmitted wirelessly to an external receiverto unburden the flash memory. In another mode, the stored digitized ECGdata can be maintained permanently until downloaded or erased to restorememory capacity. Data download or erasure can also occur before allstorage locations are filled, which would free up memory space sooner,albeit at the cost of possibly interrupting monitoring while downloadingor erasure is performed. Still other modes of data storage and capacityrecovery are possible.

The circuitry 280 of the ICM 212 can include functionality toprogrammatically select pairings of sensing electrodes when the ICM 212is furnished with three or more electrodes. In a further embodiment,multiple sensing electrodes could be provided on the ICM 212 to providea physician the option of fine-tuning the sensing dipole (or tripole ormultipole) in situ by parking active electrodes and designating anyremaining electrodes inert. The pairing selection can be made remotelythrough an inductive coupling or by the transceiver 285 via, forinstance, a paired mobile device, as further described infra. Thus, thesensing electrode configuration, including number of electrodes,electrode-to-electrode spacing, and electrode size, shape, surface area,and placement, can be modified at any time during the implantation ofthe ICM 212.

In a further embodiment, the circuitry 280 of the ICM 212 can include anactigraphy sensor 284 implemented as a 3-axis accelerometer. Theaccelerometer may be configured to generate interrupt signals to themicrocontroller 281 by independent initial wake up and free fall events,as well as by device position. In addition, the actigraphy provided bythe accelerometer can be used during post-monitoring analysis to correctthe orientation of the ICM 212 if, for instance, the ICM 212 has beeninadvertently implanted upside down, that is, with the ICM's housingoriented caudally, as well as for other event occurrence analyses.

In a still further embodiment, the circuitry 280 of the ICM 212 caninclude one or more physiology sensors. For instance, a physiologysensor can be provided as part of the circuitry 280 of the ICM 212, orcan be provided on the electrode assembly 214 with communication withthe microcontroller 281 provided through a circuit trace. The physiologysensor can include an SpO₂ sensor, blood pressure sensor, temperaturesensor, respiratory rate sensor, glucose sensor, airflow sensor,volumetric pressure sensing, or other types of sensor or telemetricinput sources.

In a yet further embodiment, firmware with programming instructions,including machine learning and other forms of artificialintelligence-originated instructions, can be downloaded into themicrocontroller's internal flash memory.

The firmware can include heuristics to signal patient and physician withalerts over health conditions or arrhythmias of selected medicalconcern, such as where a heart pattern particular to the patient isidentified and the ICM 212 is thereby reprogrammed to watch for areoccurrence of that pattern, after which an alert will be generated andsent to the physician (or other caregiver) through the transceiver 285via, for instance, a paired mobile device. Similarly, the firmware caninclude heuristics that can be downloaded to the ICM 212 to activelyidentify or narrow down a pattern (or even the underlying cause) ofsporadic cardiac conditions, for instance, atrial tachycardia (AT),atrial fibrillation (AF), atrial flutter (AFL), AV node reciprocatingtachycardia, ventricular tachycardia (VT), sinus bradycardia, asystole,complete heart block, and other cardiac arrhythmias, again, after whichan alert will be generated and sent to the physician (or othercaregiver) through the transceiver 285. For instance, an alert thatincludes a compressed ECG digitized sample can also be wirelesslytransmitted by the ICM 212 upon the triggering of a preset condition,such as an abnormally low heart rate in excess of 170 beats per minute(bpm), an abnormally low heart rate falling below 30 bpm, or AF detectedby onboard analysis of RR interval variability by the microcontroller281. Finally, a similar methodology of creating firmware programmingtailored to the monitoring and medical diagnostic needs of a specificpatient (or patient group or general population) can be used for otherconditions or symptoms, such as syncope, palpitations, dizziness andgiddiness, unspecified convulsions, abnormal ECG, transient cerebralischemic attacks and related syndromes, cerebral infarction, occlusionand stenosis of pre-cerebral and cerebral arteries not resulting incerebral infarction personal history of transient ischemic attack, andcerebral infarction without residual deficits, to trigger an alert andinvolve the physician or initiate automated analysis and follow up backat the patient's clinic. Finally, in a still further embodiment, thecircuitry 280 of the ICM 212 can accommodate patient-interfaceablecomponents, including an external tactile feedback device (not shown)that wirelessly interfaces to the ICM 212 through the transceiver 285. Apatient 210 can press the external tactile feedback device to markevents, such as a syncope episode, or to perform other functions. Thecircuitry 280 can also accommodate triggering an external buzzer 67,such as a speaker, magnetic resonator or piezoelectric buzzer,implemented as part of the external tactile feedback device or as aseparate wirelessly-interfaceable component. The buzzer 67 can be usedby the microcontroller 281 to indirectly output feedback to a patient210, such as a low battery or other error condition or warning. Stillother components, provided as either part of the circuitry 280 of theICM 212 or as external wirelessly-interfaceable devices, are possible.

The ECG front end circuit 283 of the ICM 12 measures raw subcutaneouselectrical signals using a driven reference signal, such as described inU.S. Pat. Nos. 9,700,227, 9,717,433, and 9,615,763, cited supra. Thedriven reference signal effectively reduces common mode noise, powersupply noise and system noise, which is critical to preserving thecharacteristics of low amplitude cardiac action potentials, especiallythe P wave signals originating from the atria. The ECG front end circuit283 is organized into a passive input filter stage, a unity gain voltagefollower stage, a passive high pass filtering stage, a voltageamplification and active filtering stage, and an anti-aliasing passivefilter stage, plus a reference generator. The passive input filter stagepassively shifts the frequency response poles downward to counter thehigh electrode impedance from the patient on the signal lead andreference lead, which reduces high frequency noise. The unity gainvoltage follower stage allows the circuit to accommodate a very highinput impedance, so as not to disrupt the subcutaneous potentials or thefiltering effect of the previous stage. The passive high pass filteringstage includes a high pass filter that removes baseline wander and anyoffset generated from the previous stage. As necessary, the voltageamplification and active filtering stage amplifies or de-amplifies (orallows to pass-through) the voltage of the input signal, while applyinga low pass filter. The anti-aliasing passive filter stage provides ananti-aliasing low pass filter. The reference generator drives a drivenreference signal containing power supply noise and system noise to thereference lead and is connected directly to the patient, therebyavoiding the thermal noise of the protection resistor that is includedas part of the protection circuit.

While the invention has been particularly shown and described asreferenced to the embodiments thereof, those skilled in the art willunderstand that the foregoing and other changes in form and detail maybe made therein without departing from the spirit and scope.

What is claimed is:
 1. A cardiac monitoring system, comprising: animplantable housing comprised of a biocompatible material suitable forimplantation within a living body; a pair of ECG sensing electrodes eachprovided as a wraparound electrode completely covering an end surface ofthe implantable housing between ventral and dorsal ends of the housingand a portion of each of two sides affixed to the end surface with oneof the ECG sensing electrodes forming a superior pole on a proximal endof the implantable housing and the other ECG sensing electrode formingan inferior pole on a distal end of the implantable housing to captureP-wave signals that are generated during atrial activation; an electrodepositioned on a ventral surface of the implantable housing and separatedby a periphery from one of the wraparound electrodes, wherein theperiphery is non-conductive and the electrode on the ventral surface andthe wraparound electrode function separately or the periphery isconductive and the electrode on the ventral surface and the wraparoundelectrode function as a single electrode; electronic circuitry providedwithin the implantable housing comprising: a microcontroller; and an ECGfront end circuit interfaced to the microcontroller and configured tocapture cardiac action potentials of the P-wave signals sensed by theECG sensing electrodes, and non-volatile memory electrically interfacedwith the microcontroller and operable to continuously store samples ofthe cardiac action potentials of the P-wave signals.
 2. A cardiacmonitoring system in accordance claim 1, wherein one of the ECG sensingelectrodes is dissimilar from the other ECG sensing electrode withrespect to one or more of electrode size, shape, and surface area.
 3. Acardiac monitoring system in accordance with claim 1, wherein theelectronic circuitry further comprises: a transceiver operable towirelessly interface to an external device and perform at least one ofprovide the samples of the cardiac action potentials of the P-wavesignals from the non-volatile memory to the external device and receivemodular micro program control.
 4. A cardiac monitoring system inaccordance with claim 3, wherein the microcontroller performs one ormore steps comprising: monitor the cardiac action potentials of theP-wave signals sensed by the ECG sensing electrodes for arrhythmias ofselected medical concern and wirelessly send an alert via thetransceiver upon the occurrence of one of the arrhythmias of selectedmedical concern; execute heuristics to actively identify or narrow downa pattern or underlying cause of sporadic cardiac conditions andwirelessly send an alert via the transceiver upon the occurrence of oneof the pattern and the underlying cause; and execute programmingtailored to the monitoring and medical diagnostic needs of at least oneof a specific patient, patient group or general patient population ofconditions or symptoms of medical concern and wirelessly send an alertvia the transceiver upon the occurrence of one of the conditions orsymptoms of medical concern.
 5. A cardiac monitoring system inaccordance with claim 1, further comprising: a power up sequencecomprising a check of operability and available capacity of a batteryand the non-volatile memory, wherein the microcontroller is operable toexecute the power up sequence upon the implantable housing beingimplanted into the body.
 6. A cardiac monitoring system in accordancewith claim 5, wherein the power up sequence further comprises aconfirmation of operation of the microcontroller.
 7. A cardiacmonitoring system in accordance with claim 1, further comprising: atleast one of the ECG sensing electrodes further provided to wraparoundthe ends of the implantable housing that are opposite one another.
 8. Acardiac monitoring system in accordance with claim 1, furthercomprising: a further ECG sensing electrode provided on the ventralsurface of the implantable housing between the ECG sensing electrodesprovided on the opposite ends of the implantable housing.
 9. A cardiacmonitoring system in accordance with claim 1, further comprising: anactigraphy sensor electrically interfaced with the microcontroller andoperable to sense actigraphy event occurrences based on movement of thesensor using an actigraphy event occurrence criteria and to send aninterrupt signal to the micro-controller upon sensing each of theactigraphy event occurrences.
 10. A cardiac monitoring system inaccordance with claim 1, the pair of the ECG sensing electrodes furthercomprising: an electrode shape selected from the group comprisingcircumferentially-shaped and asymmetrically-shaped electrodes.
 11. Acardiac monitoring system in accordance with claim 1, furthercomprising: a patient-interfaceable component selected from the groupcomprising an external tactile feedback device that wirelesslyinterfaces to a transceiver and an external buzzer implemented as partof the external tactile feedback device or as a separatewirelessly-interfaceable component.
 12. A cardiac monitoring system inaccordance with claim 1, wherein the implantable housing comprises ahermetically sealed implantable housing defining a rectangular shapewith rounded edges.
 13. A cardiac monitoring system in accordance withclaim 1, wherein the implantable housing comprises an external surfacehaving a length at least two times longer than a width of the externalsurface.
 14. A cardiac monitoring system in accordance with claim 13,wherein the width of each of two ends of the external surface are a samewidth.
 15. A cardiac monitoring system in accordance with claim 1,wherein the implantable housing is sized for vertical placement over atleast a portion of a heart in the body.
 16. A cardiac monitoring systemin accordance with claim 1, wherein the implantable housing comprises ahermetically sealed implantable housing defining a rectangular shapewith rounded edges with a tapered extension that is terminated on thedistal end with a further ECG sensing electrode.
 17. A cardiacmonitoring system in accordance with claim 1, further comprising: aphysiology sensor comprised within the implantable housing andelectrically coupled to the electronic circuitry with the physiologysensor operable to sense physiology of the body; and the non-volatilememory further operable to store samples of the physiology sensed by thephysiology sensor.
 18. A cardiac monitoring system in accordance withclaim 17, wherein the physiology sensor comprises one of an SpO₂ sensor,a blood pressure sensor, a temperature sensor, a respiratory ratesensor, a glucose sensor, an air flow sensor, and a volumetric pressuresensor.
 19. A cardiac monitoring system in accordance with claim 1,further comprising: one or more memory storage locations comprised inthe non-volatile memory in which samples of the ECG signals are stored,wherein, upon all of the memory storage locations being filled, themicrocontroller is operable to overwrite the samples of the ECG signalsstored in one of the memory storage locations to store the samples ofthe ECG signals acquired after the overwritten samples.
 20. A cardiacmonitoring system in accordance with claim 1, further comprising: abattery comprised within the implantable housing and which powers theelectronic circuitry.